Lathe build: 2 speed belt drive, change on the fly

[Evan]

I am trying a design of mine that provides a two speed drive for the lathe that is changable while operating and also acts as an automatic transmission under load.

It has two ratios, high speed is about a 6 to 1 and low speed is about 50 to 1. It can be switched at any time while running or stopped by moving one lever. Also, if running in high speed and the machine bogs down instead of jamming it automatically kicks into low gear with much greater torque.

The secret to the operation is a one way clutch on the compound pulley of the low gear belt train. The lever merely applies or removes tension to the high speed direct drive belt. If the high speed belt is under tension the spindle turns faster than the one way clutch and the clutch is overrun. If tension is removed from the direct drive belt or the spindle slows because of load then the clutch begins to assume the load via the compound belt drive while the direct drive belt turns at low speed so it will not be burnt while slipping.

This is not a permanent feature. I decided to try it mainly for fun and because I need to finish other parts before working on the electronics. Eventually the drive will be either via VFD or if a DC motor then via servo feedback. It is looking like the DC motor will be sufficient especially in a servo configuration. In the meantime this drive will be a convenient method of operating the machine.





Video here:
http://ixian.ca/gallery/displayimage.php?album=10&pos=0

[John Stevenson]

Aahh, you have reinvented the Myford Trilever although that had 3 speeds.

.

[small.planes]

Hmm,
It might just be me, but I dont think Id want my lathe to continue to try and snap the tool if I bogged it down
I prefer to make that decision myself.....

probably a good idea for other applications tho.

Dave

[Evan]

The tooling I use most doesn't snap. I mainly use solid carbide sticks that are about .5 x .3 x 2 inch. Even my shaper hasn't broken one of them, yet. Besides, snapping tooling is a gearhead problem.

[A.K. Boomer]

I smell rubber --- But I bet for the most part you will be in high gear most of the time, a geared (to the tension lever) half moon sheave that has clearance yet wraps around the high speed pulley side keeping the belt away would be the ticket, the old cloth V-belts allowed some le-way to be used in applications like this --- in fact many a lawn tractor used them for drive engagement --- but the all rubber contact construction of the newer belts be it serpentine or V are designed to prevent slippage --- they get warm - get gummy - and grip even better --- thats why you here them first thing in the morning after a start up, the alt. is busy paying the dividend that the starter just yanked out of the battery and the belt is cold, after a few seconds of squeeling it gets gummy enough to get the job done, even with a very lax belt there will be rubber burning in your shop as all belt carcasses have a memory of wanting to go circular --- your high speed drive pulley is preventing that.

You might have to play around with a tube of graphite for that particular belt to achieve the perfect slip/grip combo.

[Evan]

There is no sign of the main belt heating. The length of the belt in relation to the motor pulley is about 30 to 1 so it has plenty of time to cool down as it goes around at low speed. At no time is it stationary while slipping. Anyway, this is a temporary setup which is why it is a bit less than "polished" looking.

[S_J_H]

Evan,
That's a pretty slick setup!
I would not want it to have an auto shift to low function if the spindle bogged down though. But I would think in high speed that the spindle should not bog down enough to shift into low unless you were way over powering the motor with a heavy cut.
How easy is it to overload the spindle in high gear and have it shift to low?


Steve

[Evan]

It doesn't really "shift" to low. If you load the spindle so that the high speed belt is slipping the high torque low speed drive takes over. It's like it has a crummy but functional speed regulator that increase torque gradually with decreasing rpms. There is no noticable or sudden change in torque, it just gets stronger as it goes slower. In part that is due to the motor since it is running on filtered DC. The torque of a permanent magnet motor is maximum at stall so as the low speed drive takes up the slack the motor rpms go up and the motor torque goes down. It sort of evens it out.

I don't know how the high speed torque rates, I haven't tried any real machining yet. I will now that I have a usable drive system. The drive is also speed controllable via a DC output control in the form of a fan speed dimmer on the transformer input of the power supply. [old]It's running on 90VDC transformer isolated with full wave filtering with 40,000 mfd of 150vdc capacitors.[/old]

It's running isolated on 90VDC from a transformer with full wave rectification and 40,000 mfd of 150vdc filter capacitors.

Heh. No commas.

[ahidley]

How about some type of variable pulley, like a snowmobile one except with a lever???

Please make 2 as I am desperatly in need of one.

[Duffy]

Evan, That 90VDC transformer has an amorphous unobtainium core and is higgledepiggledy wound? I KNOW you intended commas after 90VDC and after isolated, but to quote Bill Cosby, "the Debil made me do it!" I am really happy that the motor worked out. Duffy